The circumference formula is , but in the unit circle , so the whole circumference is .
Pythagorean trigonometric identity
Since the hypotenuse/radius is 1, we can use the pythagorean theorem like this:
HUGE HINT ABOUT IDENTITY FUNCTIONS
Basically, if you look at the functions and , you will notice that they are the same, just shifted.
So if you look at the unit cicle, you can easily figure them out without memorizing them.